REGENERATION II3 



2. Nuclear behavior during regeneration 



Not until the primordium is already half developed (stage 5) 

 do the macronuclear nodes begin to coalesce; their fusion is not 

 so complete as in reorganization and especially division. But 

 Causin (1931) reported that regeneration is like abortive fission in 

 that the macronucleus divides, the parts later rejoining. No one 

 else has observed this. The point needs checking, in view of Yow's 

 (1958) recent work on Euplotes, showing that in regeneration two 

 ciliary anlagen are produced, just as in division, though one is 

 promptly resorbed and hence was overlooked by previous 

 investigators. 



In any event, the compacted regeneration nucleus renodulates 

 and the number of new nodes may not be the same as originally. 

 Johnson, confirming Balbiani (1889), remarked that there is in- 

 variably a slight increase in the number of nodes after renodulation. 

 The average increase was from 12 -6 to 16 macronuclear beads. If 

 the regenerating fragment was cut so as to contain few nodes to 

 start with, there was a substantial increase during regeneration. 

 Hence primordium formation in regeneration can be used for 

 correcting a decreased nucleo-cytoplasmic ratio, just as Schwartz 

 found for reorganization. This adjustment of the nuclear size to 

 the size of the fragment was confirmed by Weisz (1949a) and is in 

 accord with my own observations. 



Prowazek (1904), too, found that the number of macronuclear 

 nodes always increased during regeneration. He further stated 

 that this increase might occur even if the stentor was only diagon- 

 ally cut or injured, but he does not seem to have followed his 

 animals closely enough to exclude the possibility that an inter- 

 vening reorganization had not occurred. Evidently he believed 

 that any substantial cut or deletion of oral parts resulted in a 

 nodal increase which was also an increase in the absolute size of 

 the macronuclear material, for he stated that after the invariable 

 nuclear hypertrophy there then occurred a subsequent reduction 

 to the normal nucleo-cytoplasmic ratio. Thus in some cases he 

 found that one node of a series was absorbed, but it might have 

 fused with another. These observations should be checked 

 especially with regard to real changes in the macronuclear volume. 

 Increase in the number of macronuclear nodes following re- 

 generation was explained by Schwartz (1935) in the following 



